In the various web product industries, the demand for faster product output and increased efficiency is continually increasing. Very often, a one or two-second difference in a machine cycle can greatly impact the productivity of a system or device which manipulates, packages, and/or controls the movements of web product. In many web product industries, such a difference can be determinative as to whether or not a system or device is obsolete in light of faster and more efficient alternatives. Industries in which heightened system speed is virtually always in demand include the paper (e.g., for stacking and separating tissue paper, paper toweling, napkins, etc.), foil, textile, synthetic sheeting, and other industries. Although the following discussion focuses upon apparatuses and methods for stacking and separating interfolded web paper product, it should therefore be appreciated that the same holds true for other industries such as those just mentioned and for product which may or may not be interfolded when in stacked form.
A particular design challenge exists in the quick stacking and separation of web product in, for example, interfolding equipment. Stacking and separating processes tend to be a "bottleneck" for upstream and downstream equipment operations. Specifically, conventional stacking and separating systems and devices typically stack a stream of interfolded product upon a stacking surface which is then either lowered, shifted, or opened to separate the stacked product thereon from a new stack of product being built. Examples of stacking surfaces can be found in U.S. Pat. No. 4,874,158 issued to Retzloff, U.S. Pat. Nos. 4,770,402 and 5,299,793 issued to Couturier, and U.S. Pat. Nos. 4,700,939, 4,717,135, and 4,721,295 issued to Hathaway, all of which disclose a stack-building surface which essentially is an-elevator floor movable between an upper stack-building position and a lower stack-discharging position. U.S. Pat. No. 4,229,134 issued to Reist teaches a stack building surface which slides to drop the built stack to a surface below. As another example, U.S. Pat. No. 4,183,704 issued to Steinhart and U.S. Pat. No. 5,730,695 issued to Hauschild et al. disclose a stack-building surface which is actually a set of fork prongs or rods extending beneath and supporting the stack as it is being built.
The process of separating a completed stack from a stack which is to be built presents a speed problem for conventional systems in that time is required to pull, drop, or shift the completed stack to downstream processes. Typically, the elements and/or assemblies necessary to perform these tasks must rapidly move between a number of positions during stacking and separation operations. Nevertheless, every such movement consumes valuable time and limits system speed. One example of wasted time evident in prior art systems is the manner in which elevator-style stack building surfaces move. Conventional systems are designed so that once the stack building surface is lowered to its stack discharging position, one or more elements must complete stack discharging operations before the stack building surface can return to its elevated stack building position. The time necessary for these operations represents an inefficiency which limits the maximum operating speed of the system.
Another problem affecting the speed of conventional stacking and separating systems arises when the systems experience a jam or misfeed. In order to control the manner in which web product is stacked upon a stacking surface, it is commonly necessary to at least partly enclose the stack building surface with rails, guides, walls, or other means. Unfortunately however, this enclosed configuration leads to significant problems during and after a misfeed or jam within the enclosed area (i.e., over the stack building surface) because the area can be very difficult to clear out. Jams and misfeeds in conventional systems are therefore very time-consuming and costly.
Yet another problem experienced in conventional stacking and separating systems is not as directly related to system speed as the problems discussed above, but nevertheless significantly impacts system operations in a negative manner. Consumer demand for stacked web product having a final fold (located at the top of the completed stack, such as for a stack of packaged tissues) creates a demand for elements and assemblies which can form a final fold on the stack during the stacking and separating process. An example of such a system is described and illustrated in the Retzloff patent mentioned above. In the Retzloff patent, mentioned above (U.S. Pat. No. 4,874,158) a pair of fold fingers are mounted in a set vertical position with respect to the stack of product being built. The building stack is continually lowered as it is built, until the bottom of the stack reaches a predetermined level at which are mounted the pair of fold over fingers flanking the bottom of the stack. At a controlled time, the fold over fingers slide toward and under the stack to create a final fold in the last sheet of web product. However, the fact that the fingers are mounted in one vertical position requires this folding operation to be performed at a specific time in the stack-building operation. In some cases, the folding operation therefore limits the entire stacking and separating process, and can result in system delays.
In light of the problems of prior art systems described above, a need exists for a system and method for stacking and separating stacks of web product which can separate a completed stack from a building stack and transport the completed stack to downstream operations faster than conventional systems, which can be cleared of jams and misfeeds quickly and with minimal downtime, and which can perform final folding operations in a more flexible manner to permit faster system operations. Each preferred embodiment of the present invention achieves one or more of these results.